1. Field of the Invention
The present invention relates to a transmission power control scheme for a CDMA (Code Division Multiple Access) mobile communication system, and more particularly, to a scheme for controlling transmission powers during a soft handover in which a mobile station is simultaneously connected with a plurality of base stations in the CDMA mobile communication system, for the purpose of making a receiving quality of a reference radio channel equal to a prescribed receiving quality,
2. Description of the Background Art
In order to increase the radio channel capacity (the maximum number of simultaneous communications) in the CDMA mobile communication system, there is a need to control transmission powers so that the receiving qualities of all the radio channels are always set equal to each other.
On the other hand, in the transmission power control during the soft handover in which a mobile station is simultaneously connected with a plurality of base stations, the transmission power can be further reduced by carrying out a transmission power control for setting a receiving quality of a radio channel with the best radio channel quality (which will be referred to as a reference radio channel hereafter) equal to a prescribed reference quality (see Japanese Patent Application No. 7-77934 (1995) for detail).
In the soft handover, a plurality of base stations are connected with a combining station via wire transmission lines, where a plurality of upward transmission signals from these base stations to the combining station are selectively combined at a prescribed selective combination interval at the combining station while a plurality of downward transmission signals from these base stations to the mobile station are selectively combined at a prescribed selective combination interval at the mobile station.
When the above mentioned conventional transmission power control method disclosed in Japanese Patent Application No. 7-77934 is applied to such a soft handover using the selective combination, there arises a problem that the radio channel quality after the selective combination becomes lower than the reference quality.
Namely, from a viewpoint of the radio channel capacity, there is a need to set the transmission power control interval to be an extremely short period of time such as 1 msec, for example. On the other hand, from a viewpoint of the transmission traffic for the reliability information, it is quite difficult to set the selective combination interval equal to the transmission power control interval, and therefore the selective combination interval is set to be longer than the transmission power control interval.
FIG. 1 shows an exemplary variation in time of a receiving SIR (Signal to Interference Ratio), which is a ratio of a desired signal receiving level and an interference receiving level at each base station, for a case of using a constant maximum transmission power without the transmission power control and for a case of carrying out the conventional transmission power control.
As shown in FIG. 1, for a frame A section, when the transmission power control is OFF, the receiving SIR for a base station BS2 as indicated by a dashed line is higher so that a radio channel of this base station BS2 becomes the reference radio channel. Consequently, when the transmission power control is ON, the the receiving SIR of this base station BS2 is controlled to be equal to a reference SIR so that the base station BS2 always satisfies the reference SIR. At the same time, the receiving SIR of a base station BS1 as indicated by a solid line is also controlled to be smaller as much as the receiving SIR of the base station BS2 is made smaller. As should be apparent, at a time of the selective combination for the frame A, the received frame of the base station BS2 will be selected so that the reference SIR can be satisfied even after the selective combination.
In contrast, for a frame B section, in a section B.sub.1 up to a middle, the receiving SIR for the base station BS2 as indicated by a dashed line is higher so that the radio channel of the base station BS2 becomes the reference radio channel when the transmission power control is OFF. Consequently, when the transmission power control is ON, and consequently the receiving SIR of the base station BS2 is controlled to be equal to the reference SIR so that the base station BS2 satisfies the reference SIR while the receiving SIR of the base station BS1 is lower than the reference SIR.
Then, the receiving SIR of the base station BS1 becomes higher than that of the base station BS2 in a section B, from a middle. Here, the transmission power control interval is shorter than the frame length so that the radio channel of the base station BS1 becomes the reference radio channel from this middle point on as the receiving SIR of the base station BS1 as indicated by a solid line becomes higher from this middle point on when the transmission power control is OFF. Consequently, when the transmission power control is ON, the receiving SIR of this base station BS1 is controlled to be equal to the reference SIR so that the base station BS1 satisfies the reference SIR while the receiving SIR of the base station BS2 becomes lower than the reference SIR.
In an example shown in FIG. 1, the average SIR in the frame B section is higher for the base station BS2, so that the received frame of the base station BS2 will be selected at a time of the selective combination, but the average receiving SIR of the base station BS2 for the frame B becomes lower than the reference SIR because the receiving SIR of the base station BS2 is degraded in the section B.
Thus, there has been a problem that the communication quality becomes lower than the reference SIR after the selective combination for the frame B.
In other words, when the conventional transmission power control method of Japanese Patent Application No. 7-77934 is applied to the soft handover using the selective combination, there has been a problem that the communication quality after the selective combination cannot satisfy the reference quality at such a selective combination section in which a relationship between the receiving qualities of the base stations is reversed, because the transmission power control interval is shorter than the selective combination interval.
In particular, when the fading frequency is higher, a relationship between the receiving qualities of the base stations will be reversed more frequently within the selective combination interval, and therefore a degradation of the communication quality after the selective combination is expected to be even worse, but such a severe degradation of the communication quality is intolerable from a viewpoint of the service quality.
In addition, for the purpose of the selective combination at the combining station, the data transmission from a plurality of base stations to the combining station via a plurality of wire transmission lines is required, so that there has also been a problem concerning the increase of the wire transmission line traffic, which in turn causes the increase of the wire transmission line cost.